Exploring a Plume-Based Mass-Flux Scheme in the Boundary-Layer Gray Zone

Abstract:

We investigate the behavior of plume-based mass flux parameterizations in the gray zone of boundary layer convection, to inform the development of scale-aware and scale-adaptive parameterizations. To this end, the Eddy Diffusivity Mass Flux scheme with multiple plumes, named ED(MF)ⁿ, is implemented in a Large-Eddy Simulation (LES) model. This way, the LES is used as a non-hydrostatic larger-scale model, providing a convenient platform for investigating the behavior of parameterizations across the boundary-layer gray zone. First, as a feasibility study, a single plume is launched in every column of the LES in an offline, diagnostic way, without affecting the simulation. We find that the plumes are sensitive to the LES columns in which they rise, feeling the presence of LES clouds. A plume budget analysis shows that the plume termination height is mostly determined by the mixing term, and not so much by the buoyancy term. Then, the ED(MF)ⁿ is made interactive with the resolved flow, replacing the LES-subgrid scheme. The work performed by the scheme reduces with increasing LES resolution, due to the size-filtering applied in the ED(MF)ⁿ framework. An encouraging result is that the total specific humidity flux is conserved across the investigated range of LES resolutions, covering the gray zone between 10m (large-eddy resolving) and 10km (regional-scale). The sensitivity of the results to various model parameters is assessed.